Hi,

You will need to use custom event handler to handle document events in tree node handler. You will check the document type of the currently created document (e.g. OnBeforeInsert event) and create it under existing month or create new month document first. Below is a code sample fro handling cms.blogpost document type, so you can tak inspiration:

    // Special treatment for blog post (ensure parent month)
                        if (newdocument && (ci != null) && (ci.ClassName.ToLower() == "cms.blogpost"))
                        {
                            // Get parent node
                            node = tree.SelectSingleNode(nodeId, TreeProvider.ALL_CULTURES);
                            if (node != null)
                            {
                                // Get parent blog ID when inserting post under blog month
                                if (node.NodeClassName.ToLower() == "cms.blogmonth")
                                {
                                    node = tree.SelectSingleNode(node.NodeParentID, TreeProvider.ALL_CULTURES);
                                    if (node != null)
                                    {
                                        parentId = node.NodeID;
                                    }
                                }
                                // Get parent blog ID when inserting post under blog
                                else
                                {
                                    parentId = nodeId;
                                }

                                if (node != null)
                                {
                                    CultureInfo cultureInfo = new CultureInfo(CMSContext.PreferredCultureCode);

                                    // Prepare the data
                                    DateTime startDate = ValidationHelper.GetDateTime(formElem.BasicForm.GetFieldValue("BlogPostDate"), DateTimePicker.NOT_SELECTED);
                                    string monthName = startDate.ToString("MMMM", cultureInfo.DateTimeFormat) + " " + startDate.Year;
                                    // UpperCase first letter
                                    monthName = monthName.Substring(0, 1).ToUpper() + monthName.Substring(1);
                                    startDate = new DateTime(startDate.Year, startDate.Month, 1);

                                    // Check existing
                                    DataSet existingDS = tree.SelectNodes(CMSContext.CurrentSiteName, node.NodeAliasPath + "/%", TreeProvider.ALL_CULTURES, true, "cms.blogmonth", "BlogMonthStartingDate = '" + TableManager.GetDateTimeString(startDate) + "'", null, -1, false);
                                    if (DataHelper.DataSourceIsEmpty(existingDS))
                                    {
                                        // Create the month node
                                        node = new TreeNode("CMS.BlogMonth", tree);
                                        node.DocumentCulture = CMSContext.PreferredCultureCode;

                                        // Insert the month
                                        node.SetValue("BlogMonthName", monthName);
                                        node.SetValue("BlogMonthStartingDate", startDate);
                                        node.DocumentName = monthName;
                                        DocumentHelper.InsertDocument(node, parentId, tree);

                                        formElem.NodeId = node.NodeID;
                                    }
                                    else
                                    {
                                        // Set existing
                                        formElem.NodeId = ValidationHelper.GetInteger(existingDS.Tables[0].Rows[0]["NodeID"], 0);
                                    }
                                }
                            }
                        }

Or, you can also add similar code to \CMSModules\Content\CMSDesk\Edit\Edit.aspx.cs file (but there is a risk that it will be overwritten by future upgrade).

Best regards,
Juraj Ondrus